This invention relates to a security communication system.
Generally it is desired that information should be exchanged exclusively between those who are authorized to do so. But it is relatively easy for the outsiders to monitor the information. To prevent such a leak of information, the information has been coded or encrypted and then transmitted, and the coded information has been deciphered at the receiving station. To decipher the coded information correctly, it is necessary to carry out the coding at the transmitting station and the deciphering at the receiving station in synchronism. Hitherto, a synchronizing signal has been transmitted to the receiving station via a channel other than the information transmission channel or via the information transmission channel as pilot signal. But it is uneconomical to transmit the synchronizing signal through another channel. If the pilot signal is transmitted through the information transmission channel, the frequency band allowed to the information signal will be so limited or the dynamic range of the information signal is so limited as to degrade S/N ratio or generate jitter in the synchronizing signals, thus making it impossible to achieve an accurate synchronization between the coding and decipherring.
The frequency band allowed for general radio communication is relatively broad, for example, ranging from 300 Hz to 3 KHz. Despite this, it has been difficult to transmit coded information signal without degradation since the signal component which can be coded by the conventional security communication system has a D.C. component or an extremely low frequency component.